Clay mineral phase diagrams

The phase diagrams developed from available chemical data on natural dioctahedral minerals permits the distinction of different parageneses where the chemical components are inert, i.e., where their relative masses determine the phases produced. Other systems are possible which involve dioctahedral montmorillonites. These, or some of these, will be discussed later in conjunction with other clay minerals. 

Phase Diagrams for Zeolite—Clay Mineral Systems (Systems... 

III - GENERAL PHASE DIAGRAMS FOR ALUMINOUS CLAY MINERAL ASSEMBLAGES... 

In spite of these caveats, we will find that phase diagrams involving clays, micas, and related phases provide us with useful insights regarding such processes as weathering and mineral alteration and formation in soils and sedimentary rocks. 

Figure 9.11 Log([K+]/[H" ]) versus log[H4Si04] diagram at 25°C with plotted chemical analyses of waters in contact with clays, as reported by Aagaard and Helgeson (1983). Phase boundaries are drawn consistent with the analyses and with the illite and montmorillonite compositions proposed in the text. Agreement of the data and boundaries suggest equilibrium between the phases and support the idea that illite and montmorillonite behave as two discrete phases.AfterR.M.Garrels in C/oystfe Clay Minerals, 32 161-66, Copyright 1984.

So far we have used phase diagrams to visualize clay mineral stabilities and phase relations involving the clays in natural waters. Given the complex chemistries of mixed-layer clays in particular, geochemical computer codes offer a more rigorous way to evaluate their stabilities. The thermodynamic data bases of most of these codes list stability constants for a variety of clay minerals which, except for kaolinite, are usually of nonideal composition. Most of these stability constants have been obtained from solubility measurements and are of mixed reliability. It is appropriate to... 

Contrast the applicability and use of phase diagrams and geochemical codes for purposes of evaluating clay mineral equilibria in natural waters. 

Phase relationships in the two systems NaaO-CaO-AlgOa-SiOa and CaMgSiaOe—CaAlgSiOg have been defined, and the stability diagrams of some clay minerals in aqueous solutions determined. ... 

AI2O3 or kyanite Phases on phase diagram Major phase—mullite and minor phase—glass (can contain free Si02 ) clay since clays do not contain sufficient AI2O3 Made from 60% alumina minerals and contain some fireclay. Can be made with bauxite and clay Can be used in excess of 1700°C... 

The phases plotted are assumed to be pure and fixed in composition, and to correspond to the phases being considered in the natural system of interest. This assumption is often tenuous, particularly when the minerals involved are complex and highly variable solid solutions such as the micas and clays, which usually contain several more chemical components than can be meaningfully depicted in a two-dimensional diagram, and which may not have been chemically analyzed. 

Figure 9.16 Schematic log [M" ]/[H ]") versus log[H4Si04] diagram showing the general phase relations among primary silicates such as K-feldspar, the clays smectite, ver-miculite, illite, and kaolinite, and the mineral gibbsite.

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